Yesterday, I gave my 'antiquated' EF Prime lenses a test run with the Canon RF. using the control ring adapter.
To those who say the lenses are too slow to focus; pish tosh. Your humble bragging about selling off your EF lenses so you can get the new RF lenses is humble bragging. Of course, I have found bargains beyond belief because of such humble bragging.
I do have one RF lens, the 35mm. It's fine. Kind of noisy, but great lens. My 300mm EF L IS works great on the RF body. My 400mm NON is EF L lens works equally great.
Of all the RF lenses I have tried, I wouldn't say no to the 100-500. But no humble bragging here. I am saving for this lens, but I am not humble bragging that I am selling off my EF lenses to do this. Instead I bought an adapter ring, to continue using the excellent EF glass on the RF. And I still have my 7DmkII, that use those same EF lenses. And I have an adapter ring to use EF lenses on my Olympus OMD EM-1.